The Flat Rock Tunnel is an active railroad tunnel located on Norfolk Southern's Harrisburg Line near Manayunk, Pennsylvania, United States. The tunnel was built by the Reading Railroad for its line along the Schuylkill River. Construction of the tunnel started in 1836 and it opened in 1840. In 1858-9 the Flat Rock and Black Rock Tunnels were widened to accommodate the wider rolling stock from the Lebanon Valley Branch. The spacing between the tracks was increased from 4 ft to 6 ft. The widening of the tunnel was the first project to employ electric detonation of multiple explosive charges.